1. Field of the Invention
The present invention relates to a wireless communication device and wireless communication method whereby communication priority control is executed in a wireless LAN communication system, in particular a CSMA/CA (carrier sense multiple access with collision avoidance) system.
2. Related Background Art
In a CSMA/CA system, a terminal performs virtual carrier sensing for a random time prior to transmission to confirm that no other terminal is in communication with the base station; if another terminal is in communication with the base station, the terminal waits until this communication has been completed, and performs actual packet transmission. With this system, all terminals have equal transmission rights. “Virtual carrier sensing” means that, after the channel has been idle for a time called the IFS (Inter Frame Space), a random number is generated within a prescribed range called the CW (Contention Window) and a random time is determined based on this random number and back-off control is performed as described below within this random time. Back-off control means that control is performed wherein a calculated random number value is used as an initial value, this value is diminished with lapse of time, and actual packet transmission is performed when the value has reached 0. The IFS is prescribed in accordance with wireless LAN standard IEEE802.11, and is a fixed time for which idle detection should be performed prior to transmission; the CW is the maximum value that can be taken by the random number used in back-off: these are necessary parameters for implementation of user multiplexing. According to IEEE802.11, the minimum value CWmin and the maximum value CWmax are prescribed, and for back-off in the initial transmission a random value is calculated using the value of CWmin, and back-off is performed with the CW that is doubled every time retransmission is attempted: the value CWmax is the maximum value of CW. Communication with a plurality of terminals sharing the same channel is possible by such back-off in accordance with a random function. However, it should be noted that, with this system, there is a possibility that packet transmission may be performed by a plurality of terminals simultaneously: in this case, packet collision will occur, and packets will not be correctly received, resulting in degradation of communication quality.
A prior art technique relating to priority control in such a communication system is EDCA (Enhanced Distributed Channel Access) as specified in IEEE802.11e indicated in “ANSI/IEEE std 802.11, wireless LAN medium access control (MAC) and physical layer (PHY) specifications, 1999” (hereinafter called Document 1). In this technique, priority of four different types is assigned to the packets, and a prioritized packet transmission right is conferred on packets of high priority by shortening the transmission waiting times such as IFS, CWmin, or CWmax referred to above. In this way, relative priority control can be achieved in a communication environment in which applications of various types such as voice and data are present together. In HCCA (HCF controlled channel access), which is defined in IEEE802.11e likewise described in the above Document 1, a polling frame containing channel use opportunity (TXOP) information is transmitted to a wireless communication device by a base station, and if a terminal receives such a polling frame, the terminal obtains priority rights without being subjected to back-off.
In an improvement to the prior art technology, for example as described in Japanese Patent Application Laid-open Gazette No. 2005-12725 (hereinafter called Document 2), the base station communicates a parameter, relating to transmission waiting time, that is dependent on the amount of traffic in the entire cell, to all of the terminals, and every time the terminals perform transmission, the probability of packet collision is controlled by performing transmission in accordance with this parameter, in order to obtain smooth communication. Also, in Japanese Patent Application Laid-open Gazette No. 2005-184522 (hereinafter called Document 3), smooth communication by all of the terminals is achieved by arranging to perform transmission solely at a “transmission allowed time-point” that is allocated within a prescribed time period that is repeated periodically.